TEMPO, 2,2,6,6-tetramethylpiperidnyl-1-oxy, is a weak surfactant exhibiting a reversible redox activity: a one-electron oxidation to its oxonium cation. In the course of our earlier work (ref. Wu et al. in J Am Chem Soc 127:4490–4496, 2005; Glandut et al. in J Phys Chem B 110:6101–6109, 2006; Glandut et al. in Langmuir 22:10697–10704, 2006), we developed a full understanding of TEMPO’s electrochemistry at line microband electrodes. In these experiments TEMPO diffuses to the line electrode residing in the plane of the air/water interface in two coupled media, bulk aqueous phase with D = 7.7 × 10−6 cm2/s and along the 2D air/water interface with at least an order of magnitude greater surface diffusion constant, Dsurf. The magnitude of the TEMPO oxidation current depends jointly on Dsurf and on the rate of surface partitioning expressed by the desorption rate constant, kdes. The population of TEMPO partitioned to the air/water interface is largely unsolvated and couples to the aqueous solution by hydrogen bonding to predominately one water molecule. Our experimental methodology allows us to simultaneously determine Dsurf and kdes by recording TEMPO voltammetric curves with line microband and barrier microband electrodes. In this report, we present a new methodology of producing and characterizing barrier microband electrodes using vapor-deposited SiO and introduce additional measures such as aspiration of the air/water interface designed to substantially reduce if not eliminate negative error due to surface impurities. These investigations generated a more accurate value of Dsurf of 1.0 ± 0.3 × 10−4 cm2/s which we discuss in terms of the dynamic properties of water in the air/water interfacial region.

This correspondence presents a new strategy for detecting biological molecules that relies on competitive exchange interactions of an analyte with two-component molecular tethers attaching superparamagnetic microspheres (4 μm in diameter) to a sensor surface. The individual tethers consist of an antibody−antigen complex and are designed to selectively detect antigenic proteins in a sensitive reagentless fashion. In order to impart a driving force to the otherwise free energy neutral antibody−antigen exchange equilibrium, a small mechanical force of ∼10 pN was applied to stretch the antibody−antigen tethers using a massively parallel magnetic tweezers device. The experimental work was carried out with human cardiac troponin I. This serum heart attack marker was used as an example of analytes of credible relevance to biomedical diagnostics. The initial results illustrate the functioning of a cardiotroponin sensor and offer a preliminary estimate of its sensitivity of 16 pM.

The lateral diffusion constants of two water insoluble redox surfactants, 4-alkaneamide derivatives of 2,2,6,6-tetramethyl-1-piperidynyloxy radical (CnTempo where n = 14 or 18) were measured using 2D voltammetry with 500 μm long line electrodes positioned in the plane of the air/water interface. In order to extend these measurements into a region of low surface concentrations, we first examined the line micro-band electrodes as well as the stability of the Tempo surfactants on the water surface. Only C18Tempo proved to be sufficiently insoluble in water to form stable monolayer films over sufficiently long periods of time to assure reproducible measurements in the range of mean molecular areas (MMA) of 50–750 Å2/molecule. In this range, its diffusion constant increases linearly with MMA. At MMA of ca. 450 Å2/molecule, this dependence becomes significantly weaker suggesting proximity of a plateau region with a D of ca. 1.5 × 10−5 cm2/s. The orientation of C18Tempo on the water surface in a compressed monolayer at 52 Å2/molecule was determined by X-ray reflectivity using a synchrotron source. Only the amide group of C18Tempo appears to be immersed in the aqueous subphase while the alkane chain and the piperidine ring are located above the water surface pointing upwards.